The Untersberg is a heavily karstified massif at the Austrian-German border south of the City of Salzburg. Tracer tests conducted in the 1960s and 1970s showed that large parts of this massif are drained to the Fürstenbrunn spring, the main karst spring in the area. A deep karst flow system was previously postulated below this spring assuming that karst water infiltrates directly into the porous aquifer of the Salzburg Basin (Brandecker, 1974).Here we present results from a hydrological study shedding new light onto the karst water flow system of the Untersberg and the adjacent porous aquifer. We analysed a series of springs at Untersberg as well as a number of wells in the basin. The mean alti- 18tude of the spring's catchments was assessed using a δ O altitude gradient of -0.14 ‰/ 100 m, calculated from precipitation data. A mean residence time of ca. 0.4 yr was obtained for water emerging at the Fürstenbrunn spring. In addition to the highly constant water temperature and electrical conductivity of this spring water, this rather long residence time underscores the unusual behaviour of the Fürstenbrunn spring compared to large karst springs in other karst regions of the Northern Calcareous Alps. Monito- 18ring wells near the Berchtesgadener Ache show the same mean δ O values as the latter and also a very similar pattern throughout the year confirming the infiltration of this river into the porous aquifer. Further towards the basin, rainfall is the dominant source of recharge. Neither physical nor hydrochemical nor stable isotope data point towards karst water infiltration into the porous aquifer. In addition, the data of this study show that large fluctuations of the groundwater body in the basin also occur in winter when the discharge from the Untersberg karst system is very low.Der Untersberg ist ein stark verkarsteter Gebirgsstock an der deutsch-österreichischen Grenze südlich der Stadt Salzburg. Markierungsversuche in den 1960er und 1970er Jahren zeigten, dass die Fürstenbrunner Quelle, die die bedeutendste Karstquelle des Untersberges ist, große Teile dieses Karstmassivs entwässert. Darüber hinaus wurde unterhalb dieser Quelle ein "tiefer Karst" und somit eine Infiltration von Karstwasser in den Porenaquifer des Salzburger Beckens angenommen (Brandecker, 1974